1. Field of the Invention
The present invention relates to (ethylene, vinyl acetal) copolymers and to lithographic printing plate precursors including such copolymers.
2. Description of the Related Art
Lithographic printing typically involves the use of a so-called printing master such as a printing plate which is mounted on a cylinder of a rotary printing press. The master carries a lithographic image on its surface and a print is obtained by applying ink to said image and then transferring the ink from the master onto a receiver material, which is typically paper. In conventional lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e. ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e. water-accepting, ink-repelling) areas. In so-called driographic printing, the lithographic image consists of ink-accepting and ink-adhesive (ink-repelling) areas and during driographic printing, only ink is supplied to the master.
Lithographic printing masters are generally obtained by the image-wise exposure and processing of an imaging material called plate precursor. The coating of the precursor is exposed image-wise to heat or light, typically by means of a digitally modulated exposure device such as a laser, which triggers a (physico-)chemical process, such as ablation, polymerization, insolubilization by cross-linking of a polymer or by particle coagulation of a thermoplastic polymer latex, solubilization by the destruction of intermolecular interactions or by increasing the penetrability of a development barrier layer. Although some plate precursors are capable of producing a lithographic image immediately after exposure, the most popular plate precursors require wet processing since the exposure produces a difference of solubility or of rate of dissolution in a developer between the exposed and the non-exposed areas of the coating. In positive working plates, the exposed areas of the coating dissolve in the developer while the non-exposed areas remain resistant to the developer. In negative working plates, the non-exposed areas of the coating dissolve in the developer while the exposed areas remain resistant to the developer. Most plates contain a hydrophobic coating on a hydrophilic support, so that the areas which remain resistant to the developer define the ink-accepting, printing areas of the plate while the hydrophilic support is revealed by the dissolution of the coating in the developer at the non-printing areas.
Many lithographic printing plates contain polymeric binders such as phenolic resins which can be baked in order to increase the run length on the press. Over the last few years, printing plate materials which provide a high run length without baking have become more popular because the post-bake oven can be eliminated leading to reduced energy consumption and less floor space. The trend towards higher printing speeds on web presses and the use of recycled paper require plate coatings that are characterized by a high abrasion resistance. Unbaked phenolic resins such as novolac, resol or poly(vinyl phenol) have a poor abrasion resistance and cannot provide a high run length in such conditions.
In the prior art, the run length of lithographic printing plates based on phenolic resins has been improved by chemical modification of such binders. Examples thereof are described in for example WO 99/01795, EP 934 822, EP 1 072 432, U.S. Pat. No. 3,929,488, EP 2 102 443, EP 2 102 444, EP 2 102 445 and EP 2 102 446. Phenolic resins have also been mixed with or replaced by other polymers such as poly(vinyl acetal) resins in order to improve the abrasion resistance of the coating. Suitable poly(vinyl acetal) resins are described in U.S. Pat. Nos. 5,262,270; 5,169,897; 5,534,381; 6,458,511; 6,270,938; WO 2001/9682; EP 1 162 209; U.S. Pat. Nos. 6,596,460; 6,458,503; 6,783,913; 6,596,456; WO 2002/73315; WO 2002/96961; WO 2003/79113; WO 2004/20484; EP 1 627 732; WO 2007/17162; WO 2008/103258; WO 2009/5582; U.S. Pat. Nos. 6,255,033; 6,818,378; 6,541,181; WO 2009/85093; US 2009/4599; WO 2009/99518; US 2006/130689; US 2003/166750; U.S. Pat. No. 5,330,877; US 2005/3296; U.S. Pat. No. 8,084,189; WO 2007/3030; US 2009/0291387; US 2010/0047723 and US 2011/0059399.
U.S. Pat. No. 8,048,609 discloses a positive-working radiation-sensitive imageable element including a coating including a water-insoluble poly(vinyl hydroxyaryl carboxylic acid ester) having cyclic imide groups on some of the pendant hydroxyaryl carboxylic acid ester groups which provide to the imageable element an improved chemical resistance.
WO 2009/005582 discloses a radiation-sensitive composition including a radiation-absorbing compound, a developability-enhancing composition and an alkaline-soluble polyvinyl acetal resin comprising two different acetal groups, a first group having an optionally substituted phenol, naphthol or antracenol group and a second group having an optionally substituted naphthol group, which exhibits an improved chemical resistance.
U.S. Pat. No. 5,169,898 discloses a photosensitive composition with an improved solubility in an alkaline developing solution and which includes an acid substituted ternary acetal polymer in combination with a diazo resin.
U.S. Pat. No. 7,544,462 discloses a radiation sensitive composition comprising an alkali-soluble polymer including a phenolic resin or a polyvinyl acetal resin and a basic N-containing developability-enhancing composition, which exhibits an improved presslife even in the presence of aggressive press chemicals. WO 2004/081662 describes the use of various polyvinyl acetal resins in combination with developability-enhancing compounds in positive-working compositions and elements.
Poly(vinyl acetal) resins are prepared in the art by actualization of poly(vinyl alcohol) with aldehydes. Poly(vinyl acetals) used for lithographic printing plate coatings typically comprise both a hydrophobic acetal moiety, which provides the ink-acceptance, and an hydroxyl substituted aromatic acetal moiety, which produces the solubility differentiation in an alkaline developer upon exposure.
Such poly(vinyl acetal) resins are typically prepared by the actualization of poly(vinyl alcohol) with a mixture of aldehydes, e.g. an aliphatic aldehyde such as butyraldehyde mixed with a phenolic aldehyde such as hydroxybenzaldehyde. The physical and chemical properties of such poly(vinyl acetal) resins are highly dependent on the degree of acetalysation, the ratio of the aliphatic and the aromatic acetal moieties, the stereochemistry and the random or block nature of the acetal resin. Small shifts in process conditions during the preparation of the known acetal resins may produce significant differences in the structure of the obtained resin and thus in significant differences of their properties. For example, incomplete dissolution of the poly(vinyl alcohol) reagent may lead to an irreproducible degree of conversion, i.e. a lack of control of the composition of the final product. Also the competition and the transacetylisation which often occurs between the mixed aldehyde reagents is difficult to control so that the right balance between the hydrophobicity of the resin and its solubility in an alkaline developer cannot always be obtained.
Furthermore, the lithographic properties of the hydrophobic image areas and the hydrophilic non-image areas of a printing plate determine the quality of the prints. The greater the difference between these two properties the better the quality of the plate. A measure of this difference in properties is the so-called lithographic contrast between image and non-image parts. By applying developing solutions with an enhanced activity—i.e. by applying more aggressive developers—this contrast can be improved. However, it is often observed that during consecutive development steps, more aggressive development solutions result in a higher solid/imaging material loss, a higher loss of fine image details and/or undercutting (undercutting means that at the edges of non-exposed areas the underlying part of the layer is partially removed while the upperlying part of the layer is still present, resulting in smaller printing dot areas when the overhanging part of the upperlying part at the edge disappears). In addition, highly alkaline, more aggressive developers containing silicates and/or metasilicates are effective to remove the imaged portions of the coating, however, they can react undesirably with aluminum substrates and generate toxic waste solutions that create problems for disposal. By adding solubility enhancers to developer solutions, the solubility of the exposed parts in the developer solution will be enhanced, however, also the non-exposed parts will solubilize more easily. This often results in a lower contrast and a higher solid loss. In conclusion, there is a need for coatings having an improved solubility and/or lithographic contrast in less aggressive and more environmentally acceptable developers. At the same time, the lithographic printing plate should be sufficiently resistant against application of a variety of treating liquids or in other words, should have a high chemical resistance. Indeed, before, during and after the printing step, a lithographic printing plate is in general treated with various liquids such as for example ink and/or fountain solutions or plate treating liquids for further improving the lithographic properties of the image and non-image areas. Such liquids are applied for example to improve the hydrophilic properties of the non-image areas and to protect, restore or even enhance the hydrophobicity of the image areas. It is essential that these fluids do not deteriorate the image and/or the non-image areas throughout and well after their application.
In the unpublished patent application PCT/EP2013/075366 filed on Jan. 1, 2013, (ethylene, vinyl acetal) copolymers and their use in lithographic printing plate precursors are disclosed.